A lithium ion battery is a secondary battery having characteristics that the energy density thereof is high, and therefore a relatively high voltage can be obtained, and thus, it is frequently used in small electronic equipments such as a note-type personal computer, a video camera, a digital camera, a mobile phone, and the like. It is also expected that in the future, it may be used as a power source for large apparatuses such as an electric car and a dispersed-type power supply for household use.
As a positive electrode active material for using in a lithium ion battery, composite oxides of lithium and transition metals such as LiCoO2, LiNiO2 and LiMnO2 having a layered structure and LiMn2O4 having a spinel structure are representative. Various studies are being performed in order to improve the properties required for the positive electrode active material such as a capacity density, charge and discharge cycle durability, and safety.
In developing a novel positive electrode active material, establishing a means by which the properties of the positive electrode active material can be easily evaluated is indispensable. So far, attempts to improve the positive electrode active material has been done by evaluating the properties thereof on the basis of the infrared absorption spectrum obtained by FT-IR. According to this method, the properties of the positive electrode active material can be evaluated without actually assembling the lithium ion battery and determining the battery's properties, and thus it is useful for shortening the term of development and simplifying the screening test.
For example, Japanese Patent Application Publication No. 2005-56602 (Patent Document 1) discloses that with regard to lithium-cobalt composite oxide powder, the properties as the material for positive electrode of a lithium secondary battery can be evaluated with high accuracy by detecting each of absorbance around 1450 cm−1 and 1030 cm−1 of wave-number with a diffused reflection type Fourier transform infrared spectrophotometer, and determining whether the ratio of said absorbencies is within a particular range or not, and that the lithium-cobalt composite oxide powder for positive electrode of the lithium secondary battery which satisfies all of high capacity density, high safety, and high charge and discharge cycle durability is obtained by controlling the ratio of the absorbencies around wave-number of 1450 cm−1 and 1030 cm−1 so that it will be 0.2 or less.
Patent Document 1 further discloses that this means that the existing amount of lithium carbonate, which is a residual alkali, is relatively small in the invention described in Patent Document 1 since each of spectrum absorbance at the wave-number of 1450 cm−1 and 1030 cm−1 indicates the existing amount of lithium carbonate and lithium cobaltate, respectively. It is also disclosed that when the residual alkali exists on the surface of the positive electrode material, oxidation reaction of the electrolyte on the positive electrode is accelerated, and as the result, a film layer with high impedance is formed on the surface of the positive electrode material, and thus, homogenous insertion and release of lithium becomes difficult, causing irreversible collapse of the crystal structure of lithium cobalate as the charge and discharge cycle proceeds.
Japanese Patent Application Publication No. 10-83815 (Patent Document 2) also describes a harmful influence caused by lithium carbonate. It is disclosed that when a large quantity of lithium carbonate remains, the lithium carbonate degrades the electrolyte in some mechanism and thus gas is generated which increases the inner pressure of the battery so that the battery will swell and the properties thereof will be degraded.
In Japanese Patent Application Publication No. 2005-289700 (Patent Document 3), the distance between peak A appeared around 570-595 cm−1 and peak B around 520-550 cm−1 (difference Δ) was noted in the infrared absorption spectrum (FT-IR) of lithium-transition metals oxide, and the lithium-transition metals oxides having the bonding structure where the difference Δ is equal to or less than 50 cm−1 are described.